To illustrate the typical architecture of a mobile network, FIG. 1 shows the structure of the known GSM mobile communications system (Global System for Mobile Communications), using abbreviations known from the context of the GSM system. The system comprises several open interfaces. The transactions relating to crossing of interfaces have been defined in the standards, in which context the operations to be carried out between the interfaces have also been largely defined. The network subsystem (NSS) of the GSM system comprises a mobile services switching centre (MSC) through whose system interface the mobile network is connected to other networks, such as a public switched telephone network (PSTN), an integrated services digital network (ISDN), other mobile networks (Public Land Mobile Networks PLMN), and packet switched public data networks (PSPDN) and circuit switched public data networks (CSPDN). The network subsystem is connected across the A interface to a base station subsystem (BSS) comprising base station controllers (BSC), each controlling the base transceiver stations (BTS) connected to them. The interface between the base station controller and the base stations connected thereto is the Abis interface. The base stations, on the other hand, are in radio communication with mobile stations across the radio interface.
The GSM network is adapted to other networks by means of the interworking function (IWF) of the mobile services switching centre. On the other hand, the mobile services switching centre is connected to the base station controllers with PCM trunk lines crossing the A interface. The tasks of the mobile services switching centre include call control, control of the base station system, handling of charging and statistical data, and signalling in the direction of the A interface and the system interface.
The tasks of the base station controller include, inter alia, the selection of the radio channel between the controller and a mobile station MS. For selecting the channel, the base station controller must have information on the radio channels and the interference levels on the idle channels. The base station controller performs mapping from the radio channel onto the PCM time slot of the link between the base station and the base station controller (i.e., onto a channel of the link). The establishment of the connection will be described in closer detail in the following.
The base station controller BSC schematically shown in FIG. 2 comprises trunk interfaces 21 and 22 through which the BSC is connected to the mobile services switching centre across the A interface on the one hand and to the base stations across the Abis interface on the other hand. The transcoder and rate adaptation unit TRAU forms part of the base station system and may be incorporated into the base station controller or the mobile services switching centre. For this reason, the unit is shown in broken line in FIG. 2. The transcoders convert speech from a digital format to another, for example convert the 64 kbit/s PCM signals arriving from the mobile services switching centre across the A interface into 13 kbit/s coded speech signals to be conveyed to the base station, and vice versa. Data rate adaptation is performed between the speed 64 kbit/s and the speed 3.6, 6, or 12 kbit/s. In a data application, the data does not pass through the transcoder.
The base station controller configures, allocates and controls the downlink circuits. It also controls the switching circuits of the base station via a PCM signalling link, thus enabling effective utilization of PCM time slots. In other words, a branching unit at a base station, which is controlled by the base station controller, connects the transmitter/receivers to PCM links. Said branching unit transfers the content of a PCM time slot to the transmitter (or forwards it to the other base stations if the base stations are chained) and adds the content of the receive time slot to the PCM time slot in the reverse transmission direction. Hence, the base station controller establishes and releases the connections for the mobile station. Multiplexing of the connections from the base stations to the PCM link(s) crossing the A interface, like the reverse operation, is carried out in switching matrix 23.
The layer 1 physical interface between the base station BTS and the base station controller BSC is in this example a 2048 kbit/s PCM line, i.e. comprises 32 64 kbit/s time slots (=2048 kbit/s). The base stations are fully under the control of the base station controller. The base stations mainly comprise transmitter/receivers providing a radio interface towards the mobile station. Four full-rate traffic channels arriving via the radio interface can be multiplexed into one 64 kbit/s PCM channel between the base station controller and the base station, and hence the speed of one speech/data channel over this link is 16 kbit/s. Hence, one 64 kbit/s PCM link may transfer four speech/data connections.
FIG. 1 also shows the transfer rates used in the GSM system. The mobile station MS transmits speech data across the radio interface on the radio channel for example at the standard rate 13 kbit/s. The base station receives the data of the traffic channel and switches it to the 64 kbit/s time slot of the PCM link. Three other traffic channels of the same carrier are also located in the same time slot (i.e., channel), and hence the transfer rate per connection is 16 kbit/s, as stated previously. The transcoder/rate adaptation unit TRAU converts the encoded digital information to the rate 64 kbit/s, and at this rate the data is transferred to the mobile services switching centre. If the transcoder/rate adaptation unit is incorporated into the mobile services switching centre, maximum advantage is gained from compressed speech in data transmission.
The implementation of transmission between the base stations and base station controllers forms an essential part of the costs of the mobile network. With a growing number of network users and an increasing number and density of base stations, the significance of efficient and economic transmission solutions is emphasized even more. For signalling, this means among other things that it must be possible to perform the requisite signalling between the base station and its controller rapidly and reliably.
Present-day mobile networks as a rule have fixed two-way signalling channels between each base station and base station controller. The use of fixed channels presupposes that the transmission channels and the associated signalling channels are planned in advance and programmed in the network elements of the transmission network. Information on which time slot is used by the base station for signalling is programmed in the base station in the start-up phase. Also the cross-connects possibly provided in the transmission network must have the corresponding switching information.
Such a solution has the drawback of needing considerable planning and maintenance for the signalling channels. Since the channels must be routed through the entire network, it is difficult to take into use new base stations on account of the required definitions relating to the entire network. Furthermore, a complex network management system is required for maintenance.
Another drawback related to signalling in the known mobile networks is the fact that base station-specific, permanently allocated signalling channels require extra capacity, since they are reserved irrespective of signalling needs. This drawback is accentuated particularly in small-capacity base stations the proportion of which in mobile networks is significantly on the increase.
When network capacity has been increased, the coverage areas of individual base stations have been reduced, whereby the traffic fluctuations at an individual base station have increased. While this being the case the radio path is no longer necessarily concentrative, it is worth-while to perform concentration in the data transfer between the base station and the base station controller. This affords considerable economic savings, as the number of transmission links and of connection ports at the base station controller may be smaller than heretofore. When the transmission between the base stations and the base station controller is concentrative, totally novel solutions are needed, however.
For signalling, this means in addition to the above-mentioned rapidity and reliability that signalling must not occupy unnecessary capacity from other traffic. Also, it is desirable that the implementation of the signalling channels would not require laborious pre-planning of the kind described above and that network changes, such as adding or removal of base stations, could be performed as easily as possible.
In addition to the above facts, it is desirable that the signalling would use 64 kbit/s channels or multiples thereof, since in that case the transmission equipment need not be capable of switching parts of time slots. A signalling channel having a minimum capacity of 64 kbit/s in practice results in a solution in which the base stations use a common signalling channel.
In the known common channel solutions, the crucial point is the allocation of a channel. Generally known access principles of common transmission media include the master-slave principle, contention principle, and transmission notice principle. The following will describe briefly what the implementation of these principles to a mobile network would mean.
In implementing the master-slave solution, one of the network elements (base station controller) serves as a master element and interrogates each slave element (base station) in turn if it has anything to transmit. When detecting a slave network element requesting a transmission turn, the master network element allocates a common channel for the use of this slave network element.
When operating on the contention principle, a base station requesting a transmission turn starts its transmission on the common channel, disregarding the other base stations. If the common channel is not currently busy, this transmission request will get through the channel to the base station controller, which starts an exchange of information with the base station. If the base station controller detects a collision or the base station does not receive a response within a predetermined time, the base station repeats its transmission.
In using the transmission notice principle, the base station controller polls the base stations jointly at given intervals as to whether they have anything to transmit. Base stations requesting transmission register, whereafter the base station controller grants them a permission to transmit in turn. Also in this method, collisions of transmission requests and recovery from these must be prepared for.
In implementing the above-described known common channel solutions, the drawback would reside particularly in the slow passage of transmission requests from the base stations. In the master-slave solution, the waiting time is directly increased as the number of base stations increases. In the contention principle, the passage of the transmission request is uncertain and successive collisions may occur, especially at network peak loads. The same problems also apply to a network employing the transmission notice principle.